1. Field of the Invention
The present invention relates to the field of power electronics. It concerns a turn-off power semiconductor device comprising
(a) a semiconductor substrate having two oppositely situated principal faces, of which one is assigned to an anode and the other is assigned to a cathode and forms a cathode surface; PA1 (b) between the anode and the cathode inside the semiconductor substrate, a layer sequence comprising an emitter layer of a first conductivity type, a first base layer of a second conductivity type opposite to the first conductivity type, and a second base layer of the first conductivity type; and PA1 (c) between the anode and the cathode inside the semiconductor substrate a multiplicity of adjacently disposed turn-off unit cells connected in parallel. PA1 (d) a multiplicity of unit cells are combined in each case into a group of cells and form a segment; PA1 (e) each segment is surrounded at its periphery in the lateral direction by peripheral short-circuit regions of the first conductivity type; and PA1 (f) the peripheral short-circuit regions are embedded in the semiconductor substrate from the cathode surface and are conductively connected to a cathode contact disposed on the cathode surface. PA1 (a) the device is formed as an MOS-controlled thyristor MCT and the unit cells are each formed as MCT cells; PA1 (b) an emitter region of the second conductivity type, which is embedded in the second base layer and to which contact is made from the cathode surface by the cathode contact, being disposed inside each MCT cell; and PA1 (e) an MOS structure, which forms a switchable short circuit between the second base layer and the cathode contact, being provided inside each MCT cell on the cathode side. PA1 (a) the lateral extension of the second base layer is limited in each case to one segment and, outside the segments, the first base layer emerges at the cathode surface; PA1 (b) the peripheral short-circuit regions are embedded in the first base layer; and PA1 (c) MOS-controlled short circuits, which make a switchable connection between the cathode contact and the first base layer and, together with the peripheral short-circuit regions, the first base layer and the emitter layer form IGBT structures, are provided inside the peripheral short-circuit regions.
Such a device in the special form of an MOS-controlled thyristor MCT is disclosed, for example, in the paper by V.A.K. Temple, IEEE Trans. Electron Devices, vol. ED-33, pages 1609-1618 (1986).
2. Discussion of Background
Important system simplifications would be feasible in the case of the application in power electronic circuits, in particular in speed-controlled motor drives, if the current control as it is presently known from the GTO could be replaced by voltage control in the case of the power semiconductors used. This transition from current to voltage control has already been completed in the case of smaller powers by the replacement of the conventional bipolar transistors with the recently developed IGBTs (Insulated Gate Bipolar Transistors).
In the case of higher powers, which essentially remain the province of thyristors, efforts to replace the GTO in a similar manner by the development of the MOS-controlled thyristor MCT (MOS Controlled Thyristor) have been under way for a fairly long time. As described in the publication mentioned at the outset, in such an MCT, a multiplicity of MOS-controlled short circuits, which short-circuit the cathode contact to the cathode-side base layer to turn the device off and are consequently able to interrupt the regenerative mechanism of the thyristor, are provided inside a conventional four-layer structure on the cathode side.
Hitherto, however, these efforts have not been very successful because it is precisely also large-area MCTs which continue to suffer from detrimental inhomogeneous current density distributions, in particular during the turn-off phase. It is also such inhomogeneities which result, precisely at the periphery of the cathode-side device structure, in current-density overshoots which may destroy the device or at least impair the normal operation.